In motor vehicles, particularly heavy trucks, there is a need for means to retard the vehicle speed, particularly on downgrades, without requiring constant brake pressure which might cause brake damage through friction. Others have sought to alleviate this problem by governing engine speed by utilizing cams to open the engine exhaust valves on the compression and power strokes. However, such systems did not alter the fuel injection cycles, and fuel continues to be delivered to the cylinder at the regular time in its cycle, whereby it simply is exhausted to the atmosphere and wasted. Moreover, with the exhaust valve open during the downstroke there is little resistance to piston downstroke and little or not retardation experienced during such stroke.